1. Field of the Invention
The invention relates to an apparatus and methods for drilling lateral boreholes from a main wellbore using a high pressure jetting hose for hydrocarbon recovery. In one of its aspects, the invention relates to an apparatus and method for controlling the speed at which a high pressure jetting hose is advanced into a producing formation on the end of a tubing string.
2. Description of Related Art
The creation of lateral (also known as “radial”) boreholes in oil and gas wells using high pressure radial jetting was first introduced in the 1980's. Various tools have been used to create a lateral borehole for the purpose of extending the “reach” of the wellbore. The most currently accepted approach involves milling holes in the wellbore casing, and then subsequently using a tubing string (usually coiled or jointed tubing) to lower a high pressure jetting hose with a nozzle on its leading end into the reservoir. The configuration of the nozzle is such that it contains more opening area in the rearward facing direction than the forward direction, resulting in a forward thrust on the nozzle that pulls the hose behind it as the lateral borehole is created.
The upper end of the more-flexible jetting hose is affixed to the lower end of the less-flexible tubing string, and it is therefore desirable to feed the tubing string into the wellbore at the same speed at which the jetting nozzle is creating a lateral borehole. If the tubing feed rate is too fast, the jetting nozzle path becomes erratic and the borehole is not straight; too slow, and the jetting nozzle creates a cavity behind itself resulting in the loss of forward thrust and a borehole that is shorter. The optimal penetration rate of the jetting nozzle, and thus the optimal rate at which the tubing is fed into the wellbore, is thus dictated by the nozzle's forward and backward jets and the thrust they create.
Historically, the tubing string used to convey the jetting hose is small diameter coiled tubing of ½″ (inch) or less. The jetting hose is typically ¼″ (inch) high-pressure hydraulic hose attached to the end of the small diameter coiled tubing. This small diameter tubing possesses sufficient sensitivity and flexibility for the operator to maintain good control over the feed-in rate from the surface. The operator uses surface gauges to compare the hanging weight of the relatively lightweight (for example, 4 ft/lb) small diameter tubing to the pressure drop at the jetting nozzle, and typical sensitivity of 25-lbs is generally available.
The prior approach using small diameter flexible coiled tubing is limited, however, in terms of depth, downhole inclination angles, utilization in flowing wells, and other problem areas. Small diameter tubing also requires its own additional tube-feeding units on the surface, in addition to the standard diameter coiled tube-feeding units usually present for other phases of the drilling operation.
Using standard size coiled tubing to advance the jetting hose during lateral borehole formation would reduce or eliminate many of the depth, strength, angle, and feed unit problems noted above. But standard coiled tubing greatly reduces sensitivity and control over the jetting hose. Because success in drilling lateral boreholes using a jetting hose is greatly dependent on the sensitivity of the measurements at the surface of the well, any reduction in the operator's ability to gauge the rate of advance of the jetting hose on the end of the tubing reduces the operator's ability to control the penetration rate at which the jetting hose advances into the formation.
Standard size coiled tubing is generally constructed from carbon or stainless steel and deformably wrapped on a powerful reel on the surface; is typically on the order of 1¼″ to 1½″ (inches) in diameter or larger; and weighs significantly more (for example, 2 lbs/ft) than the small diameter coiled tubing used in the prior art. Using standard sized coiled tubing makes it significantly more difficult to control the tubing feed rate relative to the jetting nozzle penetration rate using standard weight-versus-pressure comparisons. For example, the weight gauges for standard coiled tubing are typically in 100-lb to 200-lb increments, and are simply not sensitive enough to use the hanging weight of the tubing as a benchmark for comparison to the feed-in rate and jetting nozzle pressure drop, even by a skilled operator.
Thus the use of standard coiled tubing and other larger-diameter, stronger, deep-application hose-conveying equivalents for the tubing string (such as jointed pipe with threaded connections on either end) has been discouraged.